The present invention relates to a cathode ray tube and, in particular, to a cathode ray tube including a deflection aiding electrostatic field.
Conventional cathode ray tubes (CRTs) are widely utilized, for example, in television and computer displays. One or more electron guns positioned in a neck of a funnel-shaped glass bulb of a CRT direct a corresponding number of beams of electrons toward a glass faceplate biased at a high positive potential, e.g., 30 kilovolts (kV). The faceplate usually has a substantially rectangular shape and is generally planar or slightly curved. Together, the glass bulb and faceplate form a sealed enclosure that is evacuated. The electron gun(s) are positioned along an axis that extends through the center of the faceplate and is perpendicular thereto.
The electron beam(s) is (are) raster scanned across the faceplate so as to impinge upon a coating or pattern of phosphors on the faceplate that produces light responsive to the intensity of the electron beam, thereby to produce an image thereon. The raster scan is obtained by a deflection yoke including a plurality of electrical coils positioned on the exterior of the funnel-shaped CRT near the neck thereof. Electrical currents driven in first coils of the deflection yoke produce magnetic fields that cause the electron beam(s) to deflect or scan from side to side (i.e. horizontal scan) and currents driven in second coils of the deflection yoke produce magnetic fields that cause the electron beam(s) to scan from top to bottom (i.e. vertical scan). The magnetic deflection forces typically act on the electrons of the beam(s) only within the first few centimeters, e.g., 5-10 cm, of their travel immediately after exiting the electron gun(s), and the electrons travel in a straight line trajectory thereafter, i.e through a substantially field-free drift region. Conventionally, the horizontal scan produces hundreds of horizontal lines in the time of each vertical scan to produce the raster-scanned image.
The depth of a CRT, i.e. the distance between the faceplate and the rear of the neck, is determined by the maximum angle over which the deflection yoke can bend or deflect the electron beam(s) and the length of the neck extending rearward to contain the electron gun. Greater deflection angles provide reduced CRT depth.
Modem magnetically-deflected CRTs typically obtain a xc2x155xc2x0 deflection angle, which is referred to as 110xc2x0 deflection. However, such 110xc2x0 CRTs for screen diagonal sizes of about 62 cm (about 25 inches) or more are so deep that they are almost always provided in a cabinet that either requires a special stand or must be placed on a floor. For example, a 110xc2x0 CRT having a faceplate with an about 100 cm (about 40 inch) diagonal measurement and a 16:9 aspect ratio, is about 60-65 cm (about 24-26 inches) deep. Practical considerations of increasing power dissipation producing greater temperature rise in the magnetic deflection yoke and its drive circuits and of the higher cost of a larger, heavier higher-power yoke and drive circuitry prevent increasing the maximum deflection angle as is necessary to decrease the depth of the CRT.
A further problem in increasing the deflection angle of conventional CRTs is that the landing angle of the electron beam on the shadow mask decreases as deflection angle is increased. Because the shadow mask is as thin as is technically reasonable at an affordable cost, the thickness of the present shadow mask results in an unacceptably high proportion of the electrons in the electron beam hitting the side walls of the apertures in the shadow mask for low landing angles. This produces an unacceptable reduction of beam current impinging on the phosphor and a like decrease in picture brightness for low landing angles, e.g., landing angles less than about 25xc2x0.
Even if one were to increase the deflection angle to xc2x190xc2x0 (180xc2x0 deflection) and solve the low landing angle problem, the length of the tube neck remains a limiting factor in reducing overall tube depth.
One approach to this depth dilemma has been to seek a thin or so-called xe2x80x9cflat-panelxe2x80x9d display that avoids the large depth required by conventional CRTs. Flat panel displays, while desirable in that they would be thin enough to be hung on a wall, require very different technologies from conventional CRTs which are manufactured in very high volume at reasonable cost. Thus, flat panel displays are not available that offer the benefits of a CRT at a comparable cost. But a reduced-depth cathode ray tube as compared to a CRT need not be so thin that it could be hung on a wall to overcome the disadvantage of the great depth of a conventional CRT.
Accordingly, there is a need for a cathode ray tube having a depth that is less than that of a conventional CRT having an equivalent screen-size, and reducing the added depth owing to the length of the tube neck.
To this end, the tube of the present invention comprises a tube envelope having a faceplate and a screen electrode on the faceplate adapted to be biased at a screen potential, a source of at least one beam of electrons directed away from the faceplate, wherein the source is adapted for scanning deflection of the beam of electrons, and phosphorescent material disposed on the faceplate for producing light in response to the beam of electrons impinging thereon. At least first and second electrodes are interior the tube envelope and spaced away from the faceplate for bending the beam of electrons towards the faceplate, wherein the first electrode is relatively proximate the source and the second electrode is relatively distal the source. The first electrode is adapted to be biased at a potential substantially less than the screen potential, and the second electrode is adapted to be biased at a potential one of less than and greater than the screen potential.
According to another aspect of the invention, a display comprises a tube envelope having a faceplate and a screen electrode on the faceplate adapted to be biased at a screen potential, a source of at least one beam of electrons directed away from the faceplate, wherein the source is adapted for scanning deflection of the beam of electrons, deflection means proximate the source for scanning deflection of the beam of electrons, and phosphorescent material disposed on the faceplate for producing light in response to the beam of electrons impinging thereon. At least first and second electrodes are interior the tube envelope and spaced away from the faceplate for deflecting the beam of electrons towards the faceplate, wherein the first electrode is relatively proximate the source and the second electrode is relatively distal the source, thereby defining a volume between the faceplate and the electrodes in which the beam of electrons may be deflected, wherein the first electrode is adapted to be biased at a first potential substantially less than the screen potential, and wherein the second electrode is adapted to be biased at a second potential less than the screen potential. A source provides the first, second and screen potentials.